The present generation of high frequency (HF) and microwave signals is now mostly based on Si and GaAs devices. Due to physical limitations, these devices cannot achieve power levels higher than a few hundred watts (depending on the frequency to be amplified) in simple solid-state device configurations. Wide band gap materials (diamond, SiC, GaN, etc), in principle, allow for higher power amplification per unit gate length at microwave frequencies. This is because a larger bias voltage, and hence the voltage amplitude on the microwave signal, can be supported across the transistor channel region over which the current is modulated. In effect, the higher breakdown electric field of a wide band gap semiconductor is exploited. In microwave transistors, the ability to support high voltage is particularly desirable since, generally, power has to be transferred to a relatively high impedance (50 Ω) load.
The use of diamond in manufacturing transistors of various types has been described in, for example, JP-A-60246627, EP 0 343 963 B1 and WO 2006/117621 A1.
WO 2006/117621 A1 discloses a metal semiconductor field-effect transistor (MESFET). The MESFET is manufactured by providing a single crystal diamond material substrate having a growth surface on which further layers of diamond material can be deposited, depositing a plurality of further diamond layers on the substrate growth surface, and attaching appropriate contacts to the respective diamond layers, thereby defining a transistor structure. The further diamond layers deposited on the substrate include a boron doped interface layer (a “delta-doped” layer). Such a design presents several synthesis challenges. The main challenge is the requirement to produce nanometer-thin boron layers which transition very abruptly to an intrinsic layer (e.g. a change in B concentration from about 1015 B atoms per cm3 to about 1020 B atoms per cm3 in a few nm). Growing such boron layers (delta layers) is dependent upon a number of crucial steps including substrate surface preparation and diamond growth conditions. In addition to the synthesis challenges, certain aspects of the device design are not ideal. In particular, the holes (acting as charge carriers) are essentially localised in the vicinity of the acceptors, which leads to an increase in impurity scattering and an overall degradation in the mobility.
U.S. Pat. No. 5,506,422 discloses a diamond-based three-terminal junction device that uses a material with a wider band gap than diamond to enhance the blocking properties of the gate contact. The disclosure states that conduction from the source to the drain is confined to a boron-doped layer. The use of the wide band gap material in the gate contact is not fundamental to the operation of the device, but simply a means of enhancing its performance by reducing leakage under reverse bias. By using the boron-doped layer as the channel the device of U.S. Pat. No. 5,506,422 does not exploit the superior charge carrier properties of intrinsic diamond compared with boron doped diamond.
Vogg et al (Journal of Applied Physics, vol 96 (2004), 895-902) and Nebel et al (Diamond and Related Materials, vol 12 (2003), 1873-1876) disclose pn junction diodes made from {100} and {111} diamond with an epitaxial layer of aluminium nitride. For both orientations of diamond substrate surface, it is reported in Vogg et al that there is a significant lattice parameter mismatch (−13% and +23% for {100} and {111} respectively), and that the AlN layers have a domain structure. Nebel et al reports, “The leakage current in the reverse direction is caused by imperfections of the pn heterojunction, probably caused by dislocations”. The carrier path of the device described is across the interface from the p-type doped diamond layer to the n-type doped AlN layer. This suggests that the structure of the interface between the diamond substrate and the AlN layer is highly defective, something that would be extremely detrimental to performance of an electronic device where charge flows across the interface.
Accordingly, it is an object of the invention to provide an alternative device structure, and a method of manufacture thereof, having particular advantages in terms of device manufacture and performance. Another object of the invention is to provide an alternative device structure and a method of manufacture thereof in which the charge carriers and any ionised acceptors/donors are spatially separated.
In III-V systems such as GaAs and GaN, spatial separation of the charge carriers and ionised acceptors or donors can be achieved by modulation doping. This is facilitated by the ability to form heterostructures through alloying, i.e. the addition of one or more group III or V elements into the matrix such as In or Al to change the energy band-gap of the material, whilst retaining essentially the same crystal structure across the interface. In silicon, a group IV element, heterostructures can be formed by alloying Si with Ge, another group IV element. Alloying of diamond with Si would form SiC. As SiC has a smaller band gap than diamond, such a heterostructure formed with diamond would lead to charge carrier confinement within the SiC layer and the superior electronic properties of the diamond would not be exploited.
A heterostructure is characterised by a lattice match (that is essentially the same lattice type, essentially the same lattice orientation and with a lattice parameter that is closely matched between the two materials) across the interface of the two distinct materials, and in normal usage typically refers to the situation where there is lattice continuity and the interface is defined by a distinct change in the relative concentrations of alloy components. This does not exclude the possibility of a small difference in the lattice parameter from which could give rise to an array of “misfit dislocations” at the interface. A related concept is that of heteroepitaxial growth, where the lattices on the two sides of the boundary may be distinct, but at the interface there is a substantial match or registration between the lattices.
The publication ‘Present Status and Future Prospect of Widegap Semiconductor High-Power Devices’, Japanese Journal of Applied Physics, 45 (2006), 7565-7586 provides a useful summary of prior art. FIG. 6 of the publication discloses a GaN—AlxGa(1-x)N heterostructure in which a polar AlxGa(1-x)N layer is heteroepitaxially grown on a polar GaN layer to produce a potential well for confining electrons.